In the recent semiconductor processing technology, a challenge to higher integration of large-scale integrated circuits places an increasing demand for miniaturization of circuit patterns. There are increasing demands for further reduction in size of circuit-constructing wiring patterns and for miniaturization of contact hole patterns for cell-constructing inter-layer connections. As a consequence, in the manufacture of circuit pattern-written photomasks for use in the photolithography of forming such wiring patterns and contact hole patterns, a technique capable of accurately writing finer circuit patterns is needed to meet the miniaturization demand.
In order to form a higher accuracy photomask pattern on a photomask substrate, it is of first priority to form a high accuracy resist pattern on a photomask blank. Since the photolithography carries out reduction projection in actually processing semiconductor substrates, the photomask pattern has a size of about 4 times the actually necessary pattern size, but an accuracy which is not loosened accordingly. The photomask serving as an original is rather required to have an accuracy which is higher than the pattern accuracy following exposure.
Further, in the currently prevailing lithography, a circuit pattern to be written has a size far smaller than the wavelength of light used. If a photomask pattern which is a mere 4-time magnification of the circuit feature is used, a shape corresponding to the photomask pattern is not transferred to the resist film due to influences such as optical interference occurring in the actual photolithography operation. To mitigate these influences, in some cases, the photomask pattern must be designed to a shape which is more complex than the actual circuit pattern, i.e., a shape to which the so-called optical proximity correction (OPC) is applied. Then, at the present, the lithography technology for obtaining photomask patterns also requires a higher accuracy processing method. The lithographic performance is sometimes represented by a maximum resolution. As to the resolution limit, the lithography involved in the photomask processing step is required to have a maximum resolution which is equal to or greater than the resolution limit necessary for the photolithography used in a semiconductor processing step using a photomask.
A photomask pattern is generally formed by forming a photoresist film on a photomask blank having a light-shielding film on a transparent substrate, writing a pattern using electron beam, and developing to form a resist pattern. Using the resulting resist pattern as an etch mask, the light-shielding film is etched into a light-shielding film pattern. In an attempt to miniaturize the light-shielding film pattern, if processing is carried out while maintaining the thickness of the resist film at the same level as prior to the miniaturization, the ratio of film thickness to feature width, known as aspect ratio, becomes higher. As a result, the resist pattern profile is degraded, preventing effective pattern transfer, and in some cases, there occurs resist pattern collapse or stripping. Therefore, the miniaturization must entail a thickness reduction of resist film.
As to the light-shielding film which is etched using the resist pattern as an etch mask, on the other hand, a number of light-shielding film materials have been proposed. In practice, chromium compound films are always employed because there are known a number of findings with respect to their etching and the standard process has been established. Typical of such films are light-shielding films composed of chromium compounds necessary for photomask blanks for ArF excimer laser lithography, which include chromium compound films with a thickness of 50 to 77 nm as described in Patent Documents 1 to 3.
However, oxygen-containing chlorine-base dry etching which is a common dry etching process for chromium based films such as chromium compound films often has a capability of etching organic films to some extent. If etching is carried out through a thin resist film, accurate transfer of the resist pattern is difficult. It is a task of some difficulty for the resist to have both a high resolution and etch resistance that allows for high accuracy etching. Then, for the purpose of achieving high resolution and high accuracy, the light-shielding film material must be reviewed so as to find a transition from the approach relying only on the resist performance to the approach of improving the light-shielding film performance as well.
Also, as to light-shielding film materials other than the chromium based materials, a number of studies have been made. One example of these studies is the use of tantalum in the light-shielding film for ArF excimer laser lithography, as described in Patent Document 4.
On the other hand, it has long been a common practice to use a hard mask for reducing the load on resist during dry etching. For example, Patent Document 5 discloses MoSi2 overlaid with a SiO2 film, which is used as an etch mask during dry etching of MoSi2 with chlorine-containing gas. It is described that the SiO2 film can also function as an antireflective coating.
From the past, studies have been made on metal silicide films, especially molybdenum silicide films, which can be readily etched under etching conditions for fluorine-base dry etching that causes few damages to resist film. They are disclosed, for example, in Patent Documents 5 to 7, all of which basically use a film of silicon and molybdenum=2:1. Also, Patent Document 8 discloses a metal silicide film, which has never been applied to commercial fabrication because of practical problems. In the commercial fabrication process, commonly used chromium-base light-shielding films are improved to accommodate further miniaturization.
Photomasks relying on the super-resolution technology include halftone and Levenson phase shift photomasks. When a photomask blank is processed into a photomask, a portion of light-shielding film causing a phase shift to light is removed. In this step, selective etching between the underlying film or substrate and the light-shielding film is necessary. Since commonly used chromium-base materials are superior in this sense, few studies have been made on the application of other materials.